The electronic structure and energetic stability of A2BX6 halide compounds with the cubic and tetragonal variants of the perovskite-derived K2PtCl6 prototype structure are investigated computationally within the frameworks of density-functional-theory (DFT) and hybrid (HSE06) functionals. The HSE06 calculations are undertaken for seven known A2BX6 compounds with A = K, Rb, and Cs; and B = Sn, Pd, Pt, Te, and X = I. Trends in band gaps and energetic stability are identified, which are explored further employing DFT calculations over a larger range of chemistries, characterized by A = K, Rb, Cs, B = Si, Ge, Sn, Pb, Ni, Pd, Pt, Se, and Te; and X = Cl, Br, I. For the systems investigated in this work, the band gap increases from iodide to bromide to chloride. Further, variations in the A site cation influences the band gap as well as the preferred degree of tetragonal distortion. Smaller A site cations such as K and Rb favor tetragonal structural distortions, resulting in a slightly larger band gap. For variations in the B site in the (Ni, Pd, Pt) group and the (Se, Te) group, the band gap increases with increasing cation size. However, no observed chemical trend with respect to cation size for band gap was found for the (Si, Sn, Ge, Pb) group. The findings in this work provide guidelines for the design of halide A2BX6 compounds for potential photovoltaic applications.

Perovskite solar cells, with efficiencies of 22.1%, are the only solution-processable technology to outperform multicrystalline silicon and thin-film solar cells. Whereas substantial progress has been made in scalability and stability, toxicity concerns drive the need for lead replacement, intensifying research into the broad palette of elemental substitutions, solid solutions, and multidimensional structures. Perovskites have gone from comprising three to more than eight (CH3NH3, HC(NH2)2, Cs, Rb, Pb, Sn, I, Br) organic and inorganic constituents, and a variety of new embodiments including layered, double perovskites, and metal-deficient perovskites are being explored. Although most experimentation is guided by intuition and trial-and-error-based Edisonian approaches, rational strategies underpinned by computational screening and targeted experimental validation are emerging. In addressing emergent perovskites, this perspective discusses the rational design methodology leveraging density functional theory-based high-throughput computational screening coupled to downselection strategies to accelerate the discovery of materials and industrialization of perovskite solar cells.

In this work, we investigated several titanates with lepidocrocite-type structures (general formula AxTi1–yMyO4, with A = Na and M = Li or Mg), having potential utility as anode materials for sodium-ion batteries. First-principles calculations were used to determine key battery metrics, including potential profiles, structural changes during sodiation, and sodium diffusion energy barriers for several compositions, and were compared to experimental results. Site limitations were found to be critical determinants of the gravimetric capacities, which are also affected both by the stacking arrangement of the corrugated layers and the identity of M (Li or Mg). To explain the experimentally observed lattice parameter changes observed as a function of the state of charge, it was necessary to assume the participation of water/solvent during the sodium intercalation process. Sodium diffusion barriers were also found to vary as a function of state of charge and diffusion direction, with a spread of 0.06–1.3 eV at low sodium contents, narrowing to 0.3–0.5 eV at higher sodium contents. Based on these results, strategies for selecting and improving the performance of these electrode materials are suggested.

Computational screening based on density-functional-theory calculations reveals Ge as a candidate element for replacing Pb in halide perovskite compounds suitable for light harvesting. Experimentally, three AGeI3 (A = Cs, CH3NH3 or HC(NH2)2) halide perovskite materials have been synthesized. These compounds are stable up to 150 °C, and have bandgaps correlated with the A-site cation size. CsGeI3-based solar cells display higher photocurrents, of about 6 mA cm−2, but are limited by poor film forming abilities and oxidising tendencies. The present results demonstrate the utility of combining computational screening and experimental efforts to develop lead-free halide perovskite compounds for photovoltaic applications.

•Ab initio DFT vacancy formation energies of 37 metals computed with revTPSS functional.•Surface energy error corrected vacancy formation energies for LDA, PBE and PW91.•ErevTPSS>EPBEsol∼ELDA>EPBE>EPW91ErevTPSS>EPBEsol∼ELDA>EPBE>EPW91, where E is vacancy formation energy.We report on the results of density-functional-theory based calculations of the vacancy formation energies in metals using the revised Tao–Perdew–Staroverov–Scuseria (revTPSS) functional (Pedrew et al., 2009), which is a self-consistent semilocal meta-generalized gradient approximation functional. The motivation for this work is to determine if the improved accuracy of surface energies for revTPSS compared to local and generalized gradient approximation functionals also leads to improved vacancy formation energies since vacancies can be viewed as internal surfaces. In addition, we report on the lattice constants, cohesive energies and bulk moduli predicted by revTPSS. By comparing the vacancy formation energies and bulk properties, the performance of revTPSS is assessed against four functionals: the local spin density approximation (LSDA), Perdew, Burke and Ernzerhof (PBE), Perdew–Wang-91 (PW91), and PBE for solids (PBEsol). Using an automated computational approach, we calculate the vacancy formation energies and the macroscopic properties of 34 metal systems for the five functionals. For macroscopic properties (lattice constants, cohesive energies and bulk modulus), we find the results for revTPSS typically lie between LDA and PBE with a mean absolute percentage deviation of 1.1% and 12.1% from the experimental data for lattice constants and cohesive energies respectively. Further, it is found that revTPSS predicts higher vacancy formation energies when compared to the four other functionals surveyed. We have observed the following order for the functionals with respect to the computed vacancy formation energies, Efxc:EfrevTPSS>EfPBEsol∼EfLDA>EfPBE>EfPW91. We also consider the effects of a surface-energy error correction that has been proposed for standard LDA and GGA functionals. This correction increases the vacancy formation energies of LDA, PBE and PW91 functionals. The revTPSS computed VFEs are greater than the surface-energy-corrected PBE VFEs by a mean relative difference of 14.8%.

156 scientists from 24 countries participated in CALPHAD XLI, which was held in Berkeley, California, USA, June 3–8, 2012, with 74 morning and afternoon presentations, and 72 evening presentations. The topics covered during the conference were gathered in five categories: Modeling – Software; Kinetics – Microstructure; CALPHAD Assessments – Experiments; Ab initio; and Nuclear Fuel Materials. In this brief summary, highlights of the conference are presented with titles and abstracts of all presentations.

Proton mobilities in xenotime-structured DyPO4 have been investigated through first-principles calculations based on electronic density functional theory. The calculated mobility is shown to be highly anisotropic, consistent with the tetragonal symmetry of the xenotime crystal structure. Due to the presence of one-dimensional channels along the c-axis, the hopping rate is significantly enhanced along this direction. Specifically, the activation energy for hopping along the a- and b-axes is computed to be 0.45 eV away from aliovalent dopant impurities, while the calculated energy barrier within the channels that run along the c-axis is 0.15 eV. The corresponding hopping rates along the c-axis channels are more than 2 orders of magnitude larger than those calculated previously for the monoclinic monazite-structured LaPO4 compound. The effects of aliovalent dopants on proton migration have also been investigated, considering the case of Ca2+ substitution for Dy3+. These calculations reveal a dopant-proton binding energy of approximately 0.4 eV and an increase in the hopping barriers near the dopant by up to 0.2 eV. These dopant effects were found to be relatively localized, with minimal changes to the energetics of the protons obtained more than approximately 5 Å away from the aliovalent impurity.

The properties of electron small polarons in α-MoO3 are investigated computationally employing density-functional-theory with Hubbard-U corrections (DFT+U) and hybrid functionals (HSE06). These methods are used to compute the electronic and atomic structures of polarons localized on Mo ions, the barrier for adiabatic polaron hopping, and the magnitude of the binding energy with intercalated Li ions. The calculations establish a pronounced anisotropy in polaron mobilities, both within the bilayer sheets and across the van der Waals (vdW) gaps characteristic of the α-MoO3 structure. The lowest and highest energy barriers are found for hopping within the same bilayer plane and across the vdW gap, respectively. The binding energies between polarons and intercalated Li ions are calculated in supercells with composition Li0.028MoO3, yielding values of approximately 0.3 eV when Li ions are located in the one-dimensional channels within the bilayer sheets, and values that are approximately 0.1 eV lower in magnitude when Li resides in the two-dimensional interlayer van der Waals gaps.

The stability of thin single-crystal, internal-defect-free Fe films on Mo(110) and W(110) substrates is investigated through calculations of energetics including contributions from the misfit strain, interfacial misfit dislocations, film surface and interface. The misfit dislocation model is developed through the Peierls–Nabarro framework, employing ab initio calculations of the corrugation potential at the film/substrate interface as an input to the model. The surface and interfacial energies for pseudomorphic films are calculated as a function of film thickness from 1 to 10 layers, employing first-principles spin-polarized density-functional theory calculations in the generalized gradient approximation. First-principles calculations are also employed to obtain the Fe surface stress used in the Peierls–Nabarro model to account for the strain dependence of the surface energy. It is found that the competition between the misfit strain, misfit dislocations, film surface and interfacial energies gives rise to a driving force for solid-state dewetting of a single-crystal, internal-defect-free film, i.e., an instability of a flat film that leads to formation of thicker and thinner regions. The details of the energetics are presented to demonstrate the robustness of the mechanism. Our findings indicate that misfit dislocations and their configurations play a significant role in a morphological evolution of metallic thin films.

Computational screening based on density-functional-theory calculations reveals Ge as a candidate element for replacing Pb in halide perovskite compounds suitable for light harvesting. Experimentally, three AGeI3 (A = Cs, CH3NH3 or HC(NH2)2) halide perovskite materials have been synthesized. These compounds are stable up to 150 °C, and have bandgaps correlated with the A-site cation size. CsGeI3-based solar cells display higher photocurrents, of about 6 mA cm−2, but are limited by poor film forming abilities and oxidising tendencies. The present results demonstrate the utility of combining computational screening and experimental efforts to develop lead-free halide perovskite compounds for photovoltaic applications.